The next generation mobile communication system such as a 5G New Ratio (NR) communication system will impose a more stringent requirement for user plane latency (TR 38.913) than previous predecessors. User plane latency can be defined as the time it takes to successfully deliver an application layer packet or message from a radio protocol layer 2/3 service data unit (SDU) ingress point to a radio protocol layer 2/3 SDU egress point via a radio interface in both uplink and downlink directions, where neither the reception of a mobile device nor a base station is restricted by discontinuous reception (DRX). For ultra-reliable low latency communication (URLLC) case, the target for user plane latency would be around 0.5 ms for uplink (UL) and 0.5 ms for downlink (DL). For enhanced mobile broadband (eMBB), the target for user plane latency would be around 4 ms for UL, and 4 ms for DL.
The reduction of user plane latency could be accomplished by reducing the response time between receiving an uplink grant and transmitting uplink packets which correspond to the uplink grant. FIG. 1 illustrates conventional transmission of UL packets in response to receiving a UL grant from a downlink control info illation (DCI). According to FIG. 1, from the end (101) of a DL reception of DCI which contains a UL grant to the beginning (102) of transmissions of the corresponding UL transport blocks, there could only be a delay (TDL−>UL) of 1-2 orthogonal frequency division multiplexing (OFDM) symbol durations in order to be within the required user plane latency.
There are several drawbacks of current L2 layer processing in the current Long Term Evolution (LTE) communication system. In order to perform transport block (TB) fillings, radio link control (RLC) and medium access control (MAC) protocol data units (PDUs) are generally not generated before receiving a UL grant which is for transmitting the TB. After receiving a UL grant, the L2 layer would need to process data packets of all sizes and then determine how to full up a TB as some of the data packets that are longer would need to be segmented, and some of the data packets that are less than a TB would need to be concatenated with other data packets. However, this process is quite complex because of the current logical channel prioritization (LCP) and RLC concatenation/(re-)segmentation principles. Because quality of service (QoS) requirement could be different for different channels, each channel may need to perform a different RLC concatenation/segmentation procedure. The LCP procedure is complex and cannot commence until the UL grant has been received, and thus the current LCP procedure is unlikely to achieve the required user plane latency.
The current LCP in a LTE communication system is briefly introduced as follows. First, through RRC signaling, parameters for each logical channel could be defined, and such parameters may include not limited to ‘Priority’, ‘prioritisedBitRate’ (PBR), and ‘bucketSizeDuration’ (BSD). The parameter ‘Priority’ is an integer with lower number specifying higher priority for a logical channel. PBR specifies a constant bit rate per transmission time interval (TTI) for the logical channel. BSD specifies the upper limit of a duration of a data packet for the logical channel.
MAC maintains a variable Bj (in bits) for each logical channel j. The parameter Bj would be initialized as zero and subsequently incremented by the product as the result of PBR multiplied (×) by the duration of TTI for each TTI. If Bj>PBR×BSD, then Bj would be set to be equal to PBR×BSD. Typically, the Bj grows by the size of PBR per TTI.
The current LCP procedure could be characterized as to contain these three steps. During step 1, all the logical channels with Bj>0 are allocated resources in a decreasing priority order. In other words, a logical channel with a higher priority would be allocated with resources for transmission before a logic channel with a lower priority. If the PBR of a logical channel is set to “infinity”, the MAC entity would allocate resources for all the data that is available for transmission on the logical channel before meeting the PBR of the lower priority logical channel(s). During step 2, the MAC entity would decrement Bj by the total size of MAC SDUs served to the logical channel j during step 1. Note that the Bj can be negative since the UE would generally not segment a RLC service data unit (RCL SDU) if the whole SDU fits into the remaining resources. During step 3, if any resources remain, all the logic channels are served in a strict decreasing priority order regardless of the value of Bj.
FIG. 2 shows an example of a conventional LCP according to established RLC concatenation and segmentation principles. The example of FIG. 2 contains hypothetically three logical channels, namely, LCH 1, LCH2, and LCH3 as LCH 1 has higher priority than LCH 2 which has high priority than LCH 3. The RLC SDUs (e.g. RLC SDU 1, RLC SDU 2, etc.) stands for data packets that are not yet processed by LCP. For LCH 1, after TTI=1, B1=PBR 1 which means that RLC SDU 1 and a portion of RLC SDU 2 would need to be transmitted but not yet transmitted. After TTI=2, B1 would include the first three RLC SDUs and a portion of the fourth RLC SDU which would need to be transmitted but not yet transmitted. The same could be described for B2 of LCH 2 and B3 of LCH 3.
Suppose that after TTI=1, a UE has received an uplink grant for transmitting RLC SDUs, then the UE would look for the logical channel with the highest priority and with Bj>0 which in this example is LCH 1. Next the UE would instruct LCH 1 to start transmitting packet; however, LCH 1 would always transmit a complete packet. Even though B1 would grow to include a portion of RLC SDU 2 of LCH 1 after TTI=1, in order to avoid segmentation, the MAC of UE would instruct the UE's RLC entity to transmit the entire RLC SDU 1 of LCH 1 and the entire RLC SDU 2 of LCH 1 to be contained within a first RLC PDU.
Next, the UE would look for the logical channel having the next priority and with Bj>0 which is LCH 2 in this example. Even though B2 would grow to include a portion of RLC SDU 2 of LCH 2 after TTI=1, in order to avoid segmentation, the MAC of UE would instruct the UE's RLC entity to transmit the entire RLC SDU 1 of LCH 2 and the entire RLC SDU 2 of LCH 2 to be contained within a second RLC PDU. Next, the UE would look for the logical channel having the next priority and with Bj>0 which in this example is LCH 3. However, even though B3 has grown to contain RLC SDU 1 of LCH 3 and a portion of RLC SDU 2 of LCH 3, in this example the TB has become full and cannot contain the entirety of RLC SDU 2 of LCH 3 and thus segmentation is required. This means that the remaining portion of RLC SDU 2 of LCH 3 will be transmitted in the next TB.
However, suppose that the TB is actually able to contain both the entirety of RLC SDU 1 of LCH 3 as well as the entirety of RLC SDU 2 of LCH 3, and there are no other channels with a lower priority than LCH 3, then the UE would look to fill the TB by obtaining data from the channel with the highest priority (e.g. RLC SDU 3 of LCH 1, RLC SDU 4, and etc.).
After LCP is complete, then the UE would go ahead and make RLC PDUs and MAC PDU. The RLC PDUs would involve concatenation/segmentation of RLC SDUs and also re-segmentation of RLC data PDUs. The making of MAC PDU would involve creating a MAC header which has of one or more MAC subheaders. Each of the subheaders corresponds to each of the MAC SDUs. Afterwards, paddings of the MAC PDU may need to be performed. Based on the above described process of LCP and the making of RLC PDUs and MAC PDU, it is highly unlikely that the user plane latency can be reduced to 1 or 2 OFDM symbols.